The present invention relates to a process for producing a metal wire used in reinforecement of rubber products such as tires, hoses and conveyor belts. More particularly, the present invention relates to an effective method for improving the adhesion between rubber and a metal wire.
Embedding a variety of reinforcing meterials in rubber is one of the techniques that are extensively employed for the purpose of improving the strength and durability of rubber products. Composites of rubber and steel reinforcement are applied to automotive tires, high-pressure rubber hoses and conveyor belts. While the requirements for the quality and performance of such composites widely vary in accordance with the specific area of application, it is required in all situations that the adhesion between rubber and the steel reinforcement is increased to improve the durability of the composites.
With the rapid increase in the number of expressways being constructed, there has been a strong need to enhance the durability and steering stability at high speed of radial tires using reinforcement steel cords coated with a Cu-Zn binary alloy. To meet this need, improvement in the adhesion between steel cords and rubber is of extreme importance.
With a view to improving the adhesion between steel reinforcements and rubber, studies have been conducted with two approaches, one being directed to improvements of metal wire and the other intended to improve rubber. In the first approach, efforts have been made to attain the objective not only by optimizing the composition and thickness of the metal coating but also by performing a more stringent process control over the wire production including such steps as preliminary treatments, application of metal coating on the wire, drawing the wire into a smaller diameter, but the results of these efforts are not completely satisfactory. Especially, in the drawing step of steel wires, they are drawn to have their cross-section area reduced by at least 80% so that local separation of the coating frequently occurs, which is one of the major causes of the drop in the adhesion between steel wires and rubber. In order to solve this problem, considerable efforts have so far been made to improve the configuration and arrangement of wire drawing dies or the properties of lubricants used in the drawing operation but they have met with only limited success.
During the service of rubber products, the adhesion between metal wires and rubber can deteriorate for several reasons such as the entrance of water by way of flaws that have occurred in the rubber or, in the case of automotive tires, the penetration of salt water originating from the sodium chloride used as a deicing chemical. In order to solve this problem, it has been proposed that the metal wires are coated with a ternary alloy system consisting of a copper-zinc alloy plus a third element selected from among nickel, cobalt, tin and iron (see, for example, U.S. Pat. No. 4,226,918, U.S. Pat. No. 4,255,496, U.S. Pat. No. 4,446,198 and Japanese Patent Application No. 27925/1973). This method, however, has the disadvantage that the proposed ternary alloy systems have a lower degree of workability than binary systems and metal wires coated with such ternary alloys will experience more frequent separation of the coating during the drawing step. If such metal wires were produced in large quantities and embedded in rubber products, the initial adhesion (i.e., the adhesion developing immediately after vulcanization between the metal wires and the rubber products) would be at very low levels. Because of this serious disadvantage, the idea of coating metal wires with ternary alloy systems is difficult to commercialize.
With a view to improving the adhesion between metal wires and rubber, the present invention searched for the cause of the local separation of metal coatings from steel wires that occurs during their drawing operation and found the most probable cause. In order to identify the cause, the present inventors conducted microscopic observations of metal coatings on steel wires at the exit end of each of the dies employed in the drawing step and found that the smoothness of the surface of uncoated steel wires before drawing had effects on the strength of adhesion of metal coating on drawn wires and that metal coatings formed on steel wires with uneven surface smoothness had a greater tendency to experience local separation as a result of wire drawing. FIG. 1 is a sketch of a cross section of a prior art steel wire with a metal coating as examined with a scanning electron microscope (SEM). As FIG. 1(a) shows, the undrawn steel wire 1 serving as the substrate of a metal coating had a very uneven surface, and when it was drawn to a certain extent, local separation of the metal coating 2 occurred at some high spots of the surface irregularities.
During the course of the investigation described above, the present inventors also found that the low surface smoothness of the steel wire, or the great asperity of its surface, had already occurred before it was passed into the stage of application of the metal coating. The current practice of performing preliminary treatments for the application of metal coatings is solely directed to descaling or surface activation and is little effective for the purpose of improving the surface smoothness. Most of the steel wires that have been subjected to such preliminary treatments have an average surface roughness of at least 2 .mu.m, and between about 2 and 5 .mu.m for wire diameters of 0.5 to 3 mm, and have experienced local separation of metal coating after the wires were drawn. Ruth Giuffria et al. reported the presence of the considerable irregularities in the surface of the substrate (undrawn steel wire) of metal coatings (Ruth Giuffria et al.; Rubber Chemistry and Technology, vol. 55, 1982, pp. 513-524). However, they did not show at all how the metal coating formed on steel wires with an uneven surface would change as a result or wire drawing or what effects the surface irregularities would have on the adhesion between drawn metal wires and rubber.